The most generally used method in the United States for removing light oil from coke oven gas is to use a petroleum wash oil to scrub the coke-oven gas of light oil. The wash oil is steam-distilled (debenzolized) to remove the light oil. The benzolized wash oil (wash oil rich in light oil) is introduced at the top of the still and flows down the column countercurrent to upward flow of live steam which is introduced in the base of the still column. The mixture of steam and light oil vapor exits at the top of the column. At this point it is necessary to remove the water and any wash oil which is still present in these vapors from the light oil. One method of accomplishing this which is used in industry is to introduce the vapor mixture at the base of a small-column partial condenser (dephlegmator). Cooling water is admitted at the top of the direct-contacting rectifying dephlegmator to cool and condense by direct contact with these vapors. By regulating the volume of water the temperature conditions are controlled. A fraction, consisting largely of wash oil and higher boiling light oil compounds such as naphtalene, is discharged at the bottom along with the water. This fraction is then separated from the water by decantation. The crude light oil vapors leaving the top of the rectifier are condensed and any water present is removed by decantation.
This method of removing wash oil and water from the light oil has serious disadvantages due to the sizable quantities of dirty water produced because of the direct water contact with the light-oil vapor in the direct-contacting dephlegmator. Substantial costs are required for treating this dirty water prior to re-use or disposal into streams or the like.
Attempts have been made to eliminate this waste water disposal problem by using an indirect partial condenser in place of the direct-contact partial condenser. However, this has not proved satisfactory because of serious fouling problems in the indirect partial condenser due to buildup of high boiling components as solids on the walls of the indirect partial condenser. Shutdowns and cleaning problems were believed to be sufficiently great to make this approach impractical.